A phase shifting mask for inverting the phase of light disclosed in "IEEE TRANSACTIONS ON ELECTRON DEVICES, VOLUME ED-29, [NUMBER 12], (December 1982), published in the U.S.A., Page 1828-1836" will be described prior to the description of the present invention.
The construction and characteristics of this known phase shifting mask are shown in FIGS. 5(A) to 5(D). FIG. 5(A) is a sectional view of the phase shifting mask, FIG. 5(B) is a diagram of the amplitude of light on the phase shifting mask, FIG. 5(C) is a diagram of the amplitude of light on a wafer, not shown, and FIG. 5(D) is a diagram of the amplitude of light intensity on a wafer, not shown.
The phase shifting mask comprises a transparent glass base plate 1, shading layers 2 of chromium (Cr) or chromium dioxide (CrO.sub.2) formed selectively in areas on one of the major surfaces of the base plate 1 in a predetermined pattern, and a phase shifting layer 3 formed over every other transparent area between the adjacent shading layers 2 on the same surface.
The phase shifting layer 3 is a single-layer light-transmissive film of an exposed photoresist, SiO.sub.2 SOG or MgF.sub.2, having a thickness d meeting an expression: d=.lambda./{2(n-1)}, where .lambda. is the wavelength of light projected on the phase shifting mask and n is the refractive index of phase shifting layer 3.
When a wafer, not shown, is irradiated through the phase shifting mask with light 4 of the wavelength .lambda., the amplitude of light passed through the phase shifting layer 3 appears on the lower side thereof as shown in FIG. 5(B). That is, the difference between the light passed through the phase shifting layer 3 and the light passed through a transparent section is 180.degree. and consequently the amplitude of the light appears on the wafer as shown in FIG. 5(C) and the light intensity on the wafer is enhanced as shown in FIG. 5(D).
Thus, the light intensity on the wafer is enhanced when the phase shifting layer 3 is formed on every other transparent section of the phase shifting mask having a cyclic pattern as shown in FIG. 5(A) and thereby the contrast of an image formed on the wafer can be improved.
The functions of the phase shifting mask will be described hereinafter with reference to FIGS. 6, 7(A) and 7(B).
FIG. 6 shows a cyclic phase shifting pattern consisting of shading section, i.e., line sections (sections shaded with dots) and transparent sections, i.e., spacing sections (portions of the transparent plate with a phase of 0.degree.). Shifting layers 3 having a phase difference of 180.degree. relative to the transparent plate are formed alternately in the spacing sections.
FIGS. 7(A) and 7(B) shows the distribution of light intensity on a wafer, not shown, when the wafer is irradiated through the phase shift mask. In FIG. 7(A), curves a and b indicate the distribution of light intensity in a portion of the wafer corresponding to a central portion Y2-Y2' of the phase shifting pattern of FIG. 6 and the distribution of light intensity in a portion of the wafer corresponding to an edge portions Y1-Y1' of the phase shifting pattern, respectively.
In FIG. 7(B), a curve c indicates the distribution of maximum light intensity in a portion of the wafer corresponding to a phase shifting portion X1-X1' of the phase shifting pattern.
As is obvious from FIGS. 7(A) and 7(B), light intensity I.sub.2 at a point A on the boundary between the phase shifter and a transparent section, i.e., a shifter edge portion, is lower than light intensity I.sub.1 at a point B in the central portion B of the phase shifter.